Nitric oxide is a short-lived radical species that is involved in regulatory processes including vasorelaxation. neurotransmission, and cytotoxicity. 1 This radical is generated in biological systems form from L-arginine by either (cNOS) or inducible (iN0S) nitric oxide synthase.2 cNos is associated with normal physiological regulation with one subtype located predominantly in the vascular endothelium (cNOS) and another in the brain (nN0S). iN0S is induced as a response to cytokines or endotoxin in various tissues and is associated with host defense mechanisms. The development of inhibitors to each isoform of N0S is currently of considerable interest in pharmaceutical research. We are interested in developing radiolabeled inhibitors for probing N0S enzyme activity in vivo. Two target compounds which have been reported as extremely potent and competitive inhibitors of NOS are S-alkylisothioures 1 and S-methyl-L-thiocitrulline 2 4-6 S-ethylisothiouronium hydrochloride (EITU,K=16 nM for iN0S) is 8 times more selective for iNOS than other isoforms. S-methyl-L-thiocitrulline (MTICU. Ki=l.2 nM for nNOS) displays a ten-fold preference for nN0S compared to other isoforms. In summary we have prepared two new C-11 radiolabeled inhibitors of NOS. the in vivo biodistribution and stability of these two compounds are under investigation.